It's a reference to either:
1. A rocket launcher
2. A helicopter
But also can be used for fun.
Helicopters make a soi soi soi kinda noise so saying pharses like the one in the example explains it, the rofl is added on just to make it seem kiddish and funny.

Eg. My Roflcopter goes soi soi soi soi soi (as you spin around with a rocket launcher or just because helicopters make a soi noise)

Derived from an interracial porno on the internet. Keyword "roflcopter".

A sex position, where a women lies down doggystyle with her face and chest on the bed while arching her back. The man inserts his penis and proceeds to do 360 spins, using his hands to balance and spin himself on her ass keeping his legs in the air. All while instertion is still fully fledged. Thus, simulating the propeller blade on top of a helicopter.

It is when you are rolling around on the floor laughing that you attempt to run in circles making your legs seem like the blades of a helicopter.

STEP ONE: Lay on the ground and laugh while rolling.
STEP TWO: Start moving your legs in a forward direction, as if to walk, making sure your shoes catch enough of the floor to turn in the desired direction.
STEP THREE: Keep repeating step two until point is taken.
------When to do it-----
When something sends you into histarics you may proceed to roflcopter

A helicopter made out of ASCII art. They can turn invisible, which is why nobody sees them in the sky. No one knows how the ASCII characters can stay together, however, but people still make and fly them. Usually used for skydiving.

I took a flight in a roflcopter. All I heard was soi soi soi soi soi soi, but it was the best flight I've ever been on.